1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor substrate comprising a germanium-based epitaxial film such as germanium (Ge), silicon germanium (SiGe) or the like on a foreign substrate.
2. Description of the Related Art
While the use of SOI (Silicon-On-Insulator) substrates has previously been known as a technique of increasing the carrier mobility in semiconductor devices to enhance the processing speed, the use of GeOI (Germanium-On-Insulator) substrates has recently been proposed. The reason is that the carrier mobility in the Ge crystal is higher than that in the Si crystal, with the electron mobility being about twice higher, and the hole mobility being about four times higher, which is advantageous for designing high-speed operating semiconductor devices.
To epitaxially grow a Ge film on a foreign substrate, one technique is known which comprises depositing on a Si substrate a number of Si1-xGex layers with slightly increasing Ge concentrations, and ultimately obtaining a Ge layer not containing Si (see, R. People, “Physics and applications of GexSi1-x/Si strained layer structures”, IEEE Journal of Quantum Electronics, QE-22, 1696 (1986)). In this technique, the Ge concentration in the Si1-xGex layers is gradually increased in order to prevent lattice defects referred to as misfit dislocations, which are introduced by direct epitaxial growth of a Ge film on a Si substrate due to a difference of about 4% between the lattice constants of Si and Ge.
The resulting Ge layer is then implanted hydrogen ions, and the hydrogen ion implanted Ge layer is bonded to a support substrate such as a silicon wafer with an oxide film thereon. The bonded substrate is then subjected to a heat treatment at a temperature of not less than 400° C. and not more than 600° C., causing minute hydrogen cavities referred to as microcavities to be formed in the hydrogen ion implanted interface to delaminate the substrate, resulting in a GeOI substrate.
This technique, however, requires the epitaxial growth of Si1-xGex many times, and hence increases the manufacturing costs. Most of all, it is difficult to sufficiently reduce the lattice defects introduced in the Ge epitaxial layer. Moreover, the technique entails a heat treatment subsequent to delamination in order to increase the bonding strength of the bonding interface between the support substrate and the Ge layer, in which heat treatment is performed at a relatively high temperature of not less than 800 and not more than 900° C. Considering that the melting point of Ge is 918° C., the technique cannot be regarded as being suitable to the industrial production method.
These problems can certainly be avoided by using a Ge substrate as an epitaxial substrate. However, Ge substrates are very difficult to increase in diameter, and because Ge is a rare element, the costs can consequently be very high, making the use of Ge substrates unrealistic.